AC susceptometry and DC magnetometry are two different widely used techniques for obtaining information about the magnetic characteristics of a sample. The following provides a brief background discussion of each of these well known techniques.
Briefly, in a typical dc magnetization measurement, a value for the magnetic moment m of the sample is measured for some applied dc field H.sub.dc. The magnetic moment m is a bulk sample property and is a measure of the magnetic field generated by the sample itself. The dc or static "susceptibility" is determined by dividing the magnetization by the applied field (x.sub.dc =M/H.sub.dc). When comparing different materials (or samples of the same material having different sizes), the macroscopic quantity of interest is magnetization per unit volume (or per unit mass) of the sample.
Techniques for measuring the dc moment have long been commercialized in a variety of system products such as vibrating sample magnetometers (VSMs), force magnetometers, and SQUID magnetometers. The most commonly used dc magnetometers, such as the vibrating sample magnetometer (VSM) or SQUID magnetometer, generally use a detection coil to measure the change in magnetic flux due to the presence of a magnetized sample. If the sample does not have a permanent magnetic moment, an applied field is required.
FIG. 1 schematically illustrates a typical prior art commercial DC magnetometer. In many commercial DC magnetometers, a magnet 50 (e.g., an electromagnet or superconducting solenoid) is provided to apply a constant magnetic field H.sub.dc to the sample 52 in order to magnetize the sample to a magnetic moment (m). A detection coil 54 and associated detection circuit 56 are also provided. There is no output from the detection circuit 56 until there is a magnetic flux change within the detection coil 54 (Faraday's Law). The sample flux coupled to the detection coil 54 is commonly varied by moving the sample. In a VSM, the sample 52 is vibrated near detection coil 54. As the sample 52 moves, an ac signal is generated at a frequency determined by the sample oscillation. In a SQUID system, the sample 52 is simply passed through the detection coil 54. Typically, detection circuit 56 comprises a fluxmeter, lock-in amplifier, or SQUID electronics that is coupled to the detection coil 54 in order to measure the change in flux (i.e., the magnetic moment). A so-called "extraction technique" is known wherein the fluxmeter (detection circuit) 56 deflects in an amount proportional to the flux change (moment) as the sample is removed from the sensing coil. See Cullity, Introduction to Magnetic Materials, pages 61-81, and particularly, 65-66 (Addison-Wesley Publishing Co. 1972).
Traditionally, fluxmeters, which are essentially digital voltmeters, are used for measuring secondary coil output signals over time. However, most fluxmeters have a relatively low input impedance (which can present a problem when the sensing coils to be used have a large variable resistance). In addition, fluxmeters may present potential measuring difficulties in dealing with drifts and thermal emfs.
It is generally known to use an integrating digital voltmeter as a fluxmeter. It is apparently also known to use a digital voltmeter to measure voltages to indicate DC magnetization. For example, the Rebouillat and Sasaki references cited below each appear to teach using an integrating digital voltmeter as a fluxmeter for measuring magnetization. In addition, the Werle '142 patent (cited below) teaches a magnetic fluxmeter for measuring macro flux disturbances (e.g., caused by a ship passing by) which includes a digital voltmeter type indicator for indicating the level of a signal integrated in the analog domain. The Coley '990 patent (also cited below) appears to teach a digital voltmeter arrangement in his FIG. 4 tunneling susceptometer (see components 52-66).
When properly calibrated, the output from the VSM or SQUID magnetometer yields the value of the magnetic moment for the sample. With knowledge of the sample volume (V), the magnetization (M) can be determined. Magnetization and the dc susceptibility are thus derived quantities. Usually, the moment is measured as a function of field, and the materials' magnetization curve (i.e., m or M versus H.sub.dc) is determined (see FIG. 2) by repeating the measurement for different values of H.sub.dc. That is, in a dc magnetization measurement discrete points along some characteristic magnetization curve are measured. This permits measurement of several discrete points along the magnetization curve of the sample. If the magnetic field direction can be reversed, hysteresis curves can also be generated.
DC magnetization/susceptibility measurements are extremely useful (e.g., for high field studies and for measuring hysteresis curves). However, sometimes additional (or different) information about the magnetic properties of a sample is required that is not available from a DC type measurement. For example, there are instances in which "complex" (real and imaginary) magnetic susceptibility must be measured in order to provide more complete information about the magnetic characteristics (e.g., relaxation characteristics) of a sample. AC susceptometery provides certain information (e.g., information about such "complex" parameters) that is not available from DC magnetometry. Moreover, in the AC measurement, copper wound coils can be used to generate very small amplitude AC fields (e.g, &gt;&gt;1 Oe) without complications arising from the remanent fields associated with iron-core or superconducting magnets. This means that the AC technique is very valuable in the study of low-field magnetic characteristics of a sample.
In addition, with the capability to vary the frequency of the drive field in an AC technique, the magnetodynamics of the magnetic system can also be studied. Further, since in the AC measurement the slope of the magnetization curve is being measured, non-linear magnetization and magnetic transitions are often best studied using an AC measurement.
AC susceptometry has thus been widely used for the characterization of magnetic materials for many years. However, prior to 1988, there was no serious commercial product available and ac susceptometer usage was almost universally "build your own" (so that, for example, many ac susceptometers were "laboratory assembled" using available components). Unlike the dc technique (where actual values for the magnetic moment m are measured), changes in m (i.e., .DELTA.m) are measured in the ac technique. Thus, the ac susceptibility gives an indication of the slope (dm/dH) of the magnetization curve. This is a fundamental difference between the ac and dc measurement techniques.
The discovery of high T.sub.c superconductors led to a rapid increase in interest in magnetic measurements. High T.sub.c materials are characterized by relatively small first critical fields, H.sub.cl, and a small full penetration field, H.sub.p. Therefore, a reliable low-field magnetization measurement technique is necessary for the full magnetic characterization of these materials.
In addition, the ac measurement can be used to differentiate between inter- and intragranular current coupling, and in determining the overall quality of a superconductor. An analysis of x" can provide information about the critical current density J.sub.c of these materials via the invocation of a suitable critical state model, and this sort of analysis has contributed to a better understanding of the mechanisms of superconductivity in these compounds.
An analysis of the complex susceptibility can also provide information about relaxational processes that may be occurring in the system under study. For example, it can be used to study spin-lattice relaxation phenomena in paramagnetic compounds, domain wall movement in metamagnetic systems, and has contributed to a better understanding of spin-glass systems.
Using the ac technique, the sample is generally centered within a detection coil and exposed to an applied AC magnetic field. The magnetic moment of the sample follows the applied field. The detection circuitry is generally balanced, with matched detection coils being provided in order to null out the changing flux due to the AC excitation. As a result, the detected change in flux is related only to the change in moment of the sample as it responds to the AC field.
Using well established principles of ac susceptometry, Lake Shore Cryotronics (the assignee of the present invention) introduced its 7000 line of ac susceptometers in the fall of 1988. FIG. 2A is a schematic block diagram showing this prior art ac susceptometer developed by Lake Shore. The basic principles of operation are described in Lake Shore's application note entitled "AC Susceptibility Measurement: Its Purpose and Process" (the disclosure of which is incorporated by reference herein for the purpose of providing discussion as to the state of the art). While no sample movement is required to perform AC susceptometery measurements, a motor and associated sample movement arrangement are provided in Lake Shore's ac susceptometer to move the sample in order to increase measurement accuracy and resolution.
Needless to say, much work has been done in the past in regard to magnetic characteristic measuring techniques. The following documents relate to techniques for measuring the magnetic characteristics of a sample:
Rillo et al, "Multipurpose a.c. and d.c. Equipment for Low Temperature Magnetic and Electric Measurements of Solids" (Abstract of paper presented at S ONR Workshop in May 1991);
U.S. Pat. No. 3,528,001--Yntema
U.S. Pat. No. 3,454,875--Bol et al
U.S. Pat. No. 4,861,990--Coley
U.S. Pat. No. 2,975,360--Bell
U.S. Pat. No. 4,037,149--Foner
U.S. Pat. No. 4,005,358--Foner
U.S. Pat. No. 4,238,734--Steingroever et al
U.S. Pat. No. 4,849,695--Muller et al
U.S. Pat. No. 5,008,621--Jiles
U.S. Pat. No. 3,863,142--Werle
Goldfarb et al, "Alternating-Field Susceptometry and Magnetic Susceptibility of Superconductors" (Office of Naval Research Workshop on Magnetic Susceptibility of Superconductors and Other Spin Systems, Berkeley Springs, W.V. May 1991);
Rebouillat, "High Resolution Automatic Magnetometer Using a Superconducting Magnet: Application to High Field Susceptibility Measurements", IEEE Trans. on Magnetics, v. MAG-8, n. 3 pp. 630-33 (3 Sept. 1972);
Sasaki, "Simple Precision Fluxmeter", 76 Nuclear Instruments and Methods n. 1 pp. 100-2 (1 Dec 1969);
Edwards et al, "Magnetometer for Surface Flux Density Measurement in MPI", 29 British Journal of Non-Destructive Testing n. 5, pp. 304-306 (Sept. 1987);
Beckley et al, "Simplified Electronic Permeameter Suitable for Routine and Standards Use", 9 Measurement and Control n.10, p. T65-T70 (Oct. 1976);
Marinaccio, "Op Amp Converts DVM To Fluxmeter", 48 Electronics n.10, pp. 112-113 (May 15, 1975);
De Mott, "Integrating Fluxmeter with Digital Readout", IEEE Journal on Magnetics v. MAG-6 n.2 pp. 269-71 (2 Jun. 1970); and
Press Release, "DOWTY RFL Offers The Only Digital Fluxmeter With IEEE-488 Bus Operation", Dowty RFL Industries, Boonton N.J. (Mar. 24, 1986).
Jiles teaches a measuring device capable of measuring various magnetic parameters (e.g., fluxmeter, gaussmeter, strain indicator), and uses the same overall transducer assembly for various measurements. The Dowty RFL press release describes a digital fluxmeter which measures "both flux density and total flux".
In addition, AC susceptometers, DC magnetometers and Vibrating Sample Magnetometers are commercially available from several companies. For example:
Quantum Design Inc. of San Diego, Calif. has been marketing a DC magnetometer since 1985, and as early as 1990 announced an option for making complex ac susceptibility measurements using elements of the SQUID detection system in its dc magnetometer. The means for making the ac measurements is not clear, nor has this group been able to qualify performance characteristics. In fact, it appears that no units have actually been delivered as of the filing date of the subject application.
Cryogenic Consultants Ltd., London England has been marketing, for approximately two years, a DC magnetometer employing a SQUID detection system. This device has no ac measurement capabilities. Cryogenic Consultants also markets a vibrating sample magnetometer.
Metronique Ingenierie of Le Bourget, France (no longer a going concern) introduced a SQUID (dc) Magnetometer in December 1989. They offered an ac measurement option which consisted of a separate set of sensing coils. The customer had to place a separate ac measurement insert into the dewar (refrigerator) system.
EG&G Princeton Applied Research (PARC), Princeton N.J., long ago introduced a vibrating sample magnetometer (VSM) that makes dc magnetization measurements. This instrument in found in many installations around the United States.
Princeton Measurements Corporation of Princeton New Jersey introduced an Alternating Gradient Force Magnetometer (AGFM) about two years ago--possibly to compete against VSMs and room temperature applications for SQUID magnetometers.
Phasetrack Instruments of Santa Clara, Calif. markets an ac susceptometer on a small scale. This instrument is not capable of making dc magnetization measurements.
See also generally the following papers relating to magnetic characteristic measurement: John K. Krause and Jeffrey R. Bergen, "Understanding magnetic measurement techniques," Superconductor Industry, vol. 3, no. 4, pp. 23-26, 1990;
Jiles, Magnetization and Magnetic Materials, pages 47-68 (Chapman & Hall 1991);
Goldfarb, "Thermoremanent magnetization and superparamagnetism in nickel-manganese alloys", Ph.D. dissertation (Colorado State University 1979);
Khoder et al, "Calibration constant calculations for Magnetic Susceptibility";
Couach et al, "Study of Superconductors by AC Susceptibility", Cryogenics Vol. 25, pp. 695-99 (1985);
Goldfarb et al, "Calibration of ac susCeptometers for cylindrical specimens", Rev. Sci. Instrum. vol. 55, pp. 761-64 (1984):
Zieba et al, "Superconducting Magnet Image Effects Observed With A Vibrating Sample Magnetometer", Rev. Sci. Instrum. Vol. 54, pp. 137-45 (1983); and
Rillo et al, "On the Sensitivity of High-TC Superconducting Ceramics as Magnetic-Field Sensors", Sensors and Actuators A-Physics, Vol. 27, N1-3 pp. 775-80 (1991).
For some time there has been a desire in the field to develop a practical, cost-effective commercial instrument capable of providing both ac and dc measurement techniques. As mentioned above, for a complete study of the magnetic properties of at least certain samples it is desirable to perform both ac and dc measurements. However, despite such desire, no one in the past has developed a practical, cost-effective instrument that is capable of accurately measuring the characteristics of a sample using both ac and dc techniques.
The present applicants have developed a preferred embodiment magnetic measurement system that is versatile, highly accurate, and can measure both AC susceptibility and DC moment. The instrument provided in accordance with a presently preferred exemplary embodiment of the present invention includes various components (i.e., two oppositely wound secondary coils, a source of AC excitation current coupled to a primary winding, and a stepping motor for moving the sample between the coils) to perform AC susceptibility measurements. The instrument also includes a source of DC current that can be coupled to the primary winding. The motor and associated sampling positioning arrangement is also used to provide the motion needed for "extraction type" DC magnetization measurement. A high speed digital voltmeter monitors and records the output of the secondary coil(s). The recorded output of the digital voltmeter is numerically processed (using a computer) to yield the voltage integral indicative of magnetic moment.
Thus, a presently preferred exemplary embodiment of the present invention provides a combination of the extraction technique for DC moment measurement with the AC susceptibility measurement using a common sensing structure--all within an instrument providing state-of-the-art electronics and computer control. By way of non-limiting example, applicants' presently preferred embodiment system provides the following advantages:
A single instrument, requiring no hardware reconfiguration between measurement modes, that is capable of measuring both AC and DC magnetic response of materials;
AC and DC measurements can be made without removing sample from dewar--thereby eliminating possible errors due to changes in set-up, changes in sample condition, etc.;
Use of common components (e.g., coil assembly) permits common calibration factors to be used for both AC and DC measurements;
Calibration constants for both AC and DC measurements can be calculated from considerations of coil geometry alone (unnecessary to calibrate with standard magnetic materials--since the coil assembly is precision wound and the system has been designed to avoid extraneous effects);
Stepper motors and associated sample suspension/mounting structure used for DC magnetization extraction technique also used for moving sample during AC measurements in order to cancel out differences between the two coils;
Dual opposed secondary coils cancel magnetic field noise;
Non-zero voltage offsets within the digital voltmeter during DC measurements are cancelled during subsequent noise reduction analysis--permitting higher speed digital voltmeter data acquisition in order to minimize "dead time" between measurements and thereby increase measurement sensitivity;
AC susceptibility measurement sensitivities comparable to or greater than those achievable using SQUID magnetometers (e.g., 2.times.10.sup.-8 emu);
High dc measurement sensitivity levels (e.g., 5.times.10.sup.-5 emu) can be achieved with an effective dynamic range extending to &gt;10.sup.3 emu, comparable to Vibrating Sample Magnetometers;
Broad dynamic range (10.sup.-8 to &gt;10.sup.3) to permit a wide array of material properties to be studied;
AC susceptibility measurement over a wide range of temperatures (e.g., &lt;4.2K. to 325K.), amplitudes (e.g., 0.1 A m.sup.-1 ()0.00125 Oe) to 1600 Am.sup.-1 (20 Oe) RMS), and frequencies (e.g., 1 Hz to 10 kHz);
DC moment measurement over a wide range of temperatures and DC fields (e.g., 1.0 tesla or 5.0 tesla, plus or minus);
Capability of measuring harmonic susceptibilities, and AC and DC resistance (e.g., Hall effect, Transport J.sub.c, magnetoresistance);
For AC measurements, primary coil is driven with an AC current source so that the resultant AC field depends only on the "constancy" of the current source output--thereby eliminating complicated phase relationships dependent on measurement frequency or temperature;
Virtual elimination of effects of eddy current generation in conductive materials or generation of persistent currents in superconductive solenoids inductively coupled to the secondary coils;
Fully automated for unattended operation with data acquisition and control software that is quickly and easily tailored to address specific research requirements;
Possible to input sample parameters (e.g., volume, mass) and demagnetization factors to assure that the resultant measurement is as accurate as possible;
Wide range of materials and applications. The system is well suited for studying paramagnetic and ferromagnetic materials, amorphous alloys and diluted magnetic semiconductors, organic ferromagnets and organic superconductors (C.sub.60 compounds), conductive polymers, thin film recording media, etc.
Ability to flexibly configure a common instrument platform for AC measuring capabilities, DC measuring capabilities, or both--without sacrificing accuracy of either measurement mode.
Expandable configuration to permit purchasers to upgrade measuring system by adding additional measurement mode capabilities subsequent to purchase.
Applicants have thus developed a single instrument that incorporates both a dc moment measurement scheme and an ac susceptometer. The instrument provides a sensitive dc measurement capability which can be simply implemented, without sacrificing any of the performance characteristics of the ac measurement.
After considering a number of standard dc moment measurement schemes, applicants chose an extraction technique. Extraction refers to many variations of a basic method, but generally involves moving (extracting) a magnetized sample from within a sensing coil. The voltage induced in the coil is detected and integrated over time to yield the total flux change in the coil. The flux change is directly related to the magnetic moment of the sample. An extraction method is attractive for this application since all required experimental hardware is already in place for the ac susceptibility measurement. The only missing component is the means to detect the voltage induced in the sensing coils. After reviewing experimental requirements and instrument specifications, applicants decided to use a high speed digital voltmeter (DVM). The DVM specifications indicated that performance comparable to, or better than, a commercial fluxmeter could be achieved.
One aspect of the present invention thus provides a relatively simple arrangement to provide adc moment measurement capability in an ac susceptometer requiring minimal effort and hardware changes. Performance is comparable to many systems constructed solely as adc magnetometer. An important element to the measurement in the preferred embodiment is the use of a high speed digital voltmeter (DVM) for the signal analysis. In addition to providing the required resolution and sensitivity needed for the moment measurement, the features of the DVM can be used to add further capabilities to the system at minimal expense. For example, with the addition of a sample probe, a dc resistance measurement can now be performed.